The present invention generally relates to an optical apparatus for an electrophotographic copying machine having a plurality of magnification modes and, particularly, to a positioning device for retaining a projecting lens assembly exactly at a predetermined position appropriate to a particular magnification mode. More particularly, the present invention pertains to an optical apparatus having three or more magnification modes including a mode wherein the magnification is 1, that is, the life size reproduction mode.
In a prior art electrophotographic copying machine of a slit exposure type, a construction of which is schematically shown in FIG. 1 of the accompanying drawings, an original 2 to be copied, which is placed on a transparent support 1, is illuminated by an illuminating lamp 3 while the latter is moved together with a first reflective mirror 4 from one position shown in solid lines to another position shown in broken lines at a predetermined velocity V to successively scan the original 1 to be copied. Simultaneously with the movement of the illuminating lamp 3 together with the first reflective mirror 4, a second reflective mirror 5 is moved in a direction parallel to the direction of movement of the illuminating lamp 3 from the solid line position to the broken line position at a velocity equal to half the velocity V of movement of the illuminating lamp 3, that is, at a velocity of V/2. An image of the original 1 to be copied is transmitted to a projecting lens assembly 6 by the first reflective mirror 4 and then the second reflective mirror 5 and is subsequently reflected by third and fourth reflective mirrors 7 and 8 and then through an exposure slit 10 onto a photoreceptor surface, for example, a photoconductive outer peripheral surface of a drum 9 being rotated in a direction shown by the arrow at a peripheral velocity Vo past an exposure station.
Prior to the photoreceptor drum 9 being moved past the exposure station, the photoconductive surface of the drum 9 is electrostatically charged at a charging station by a corona charging device 19. The electrostatically charged photoconductive surface of the drum 9 is then exposed at the exposure station to light projected through the exposure slit 10 by the optical system including the projecting lens assembly 6 so that an electrostatic latent image is formed on a local surface area of the photoconductive surface of the drum 9 in a pattern corresponding to the pattern of the image of the original 1 to be copied. The electrostatic latent image is subsequently developed into a powder or toner image by exposing the photoconductive surface of the drum 9 to a developing material supplied at a developing station from a developer unit 11. The toner image can then be transferred from the photoconductive surface of the drum 9 to a sheet of final support material, for example, copying paper, which has been supplied from a paper supply unit 12 in synchronism with the rotation of the photoreceptor drum 9. This transfer of the toner image from the photoconductive surface of the drum 9 to the copying paper at a transfer station is carried out by electrically charging the copying paper by means of a transfer corona charger 14 and placing the copying paper in contact with the photoconductive surface of the drum 9.
After the transfer of the toner image from the photoconductive surface of the drum 9 to the copying paper, the photoconductive surface of the drum 9 is cleaned by a cleaning unit 17 and the residual electrostatic charges on the photoconductive surface of the drum 9 are then erased by exposing it to light from an erasing lamp 18. On the other hand, the copying paper bearing the toner image transferred from the photoreceptor drum 9 in the manner described above is separated from the photoreceptor drum 9 by a separator pawl assembly 16, then passed through a fixing station where the toner particles forming the toner image on the copying paper are fused by heat by a fixing unit 13, and is finally discharged from the copying machine.
All of the construction and the operation of the copying machine shown in FIG. 1 are well known to those skilled in the art.
Assuming that the optical system employed in the coying machine of the construction shown in FIG. 1 has a plurality of, for example, three or more, magnification modes, the first and second reflective mirrors 4 and 5 must be moved at the respective velocities V and V/2 during one of the magnification modes wherein the magnification is 1, that is, during the life size reproduction mode. However, in order to establish another one of the magnification modes, the optical system, particularly, the projecting lens assembly 6 and the third and fourth reflective mirrors 7 and 8 must be repositioned so as to satisfy the following relationships. EQU V.sub.1 .multidot..beta..sub.1 =V.sub.2 .multidot..beta..sub.2 ( 1) EQU l={f.multidot.(.beta..sub.1 -.beta..sub.2)}/(.beta..sub.1 .multidot..beta..sub.2) (2) EQU .DELTA.L=f.vertline.{(1+.beta..sub.1).sup.2 /.beta..sub.1 }+{(1+.beta..sub.2).sup.2 /.beta..sub.2 }.vertline. (3) ##EQU1## wherein l represents the distance of movement of the projecting lens assembly, .DELTA.L represents the amount of change of the conjugate length, f represents the focal length of the projecting lens assembly, .beta..sub.1 and .beta..sub.2 represent respective magnifications and V.sub.1 and V.sub.2 represent respective velocities of movement. Also, d represents the distance of displacement third and fourth reflective mirrors 7 and 8 wherein .gamma. in the equation therefor represents half angle of intersection of the optical axis as shown in FIG. 1.
In order to establish a magnification mode other than the life size reproduction mode, a magnification selector switch must be manipulated to reposition the projecting lens assembly and the third and fourth reflective mirrors in such a manner as to satisfy the above described relationships. By way of example, if the projecting lens assembly has a focal length of 280 mm, this projecting lens assembly must be moved to a position spaced about 153 mm from the position for the life size reproduction mode when a reproduction mode wherein the magnification is .times.0.647 (It is to be noted that throughout this specification and claims the term "magnification" is taken to cover both positive and negative magnifications. Negative magnifications are, of course, reductions.) is desired to be achieved.
In addition, the smaller the magnification, the more the position of the focal point of the projecting lens assembly is adversely affected by an error in positioning the projecting lens assembly to any one of the lens positions respectively corresponding to the different magnification modes. Conversely, an error in magnification may be large as the magnification becomes 1. By way of example, assuming .DELTA.L.ltoreq.0.1 m and .DELTA..beta..ltoreq.0.05, the error .DELTA.a in positioning the projecting lens assembly must be within a range of up to about 1.4 mm during the life size reproduction mode, within a range of up to 0.25 mm during the .times.0.785 magnification mode, and within a range of up to 0.17 mm during the .times.0.647 magnification mode. It will readily be seen that, during the .times.0.785 and .times.0.647 magnification modes, the projecting lens assembly is required to be repositioned as precisely as possible as compared with the repositioning of the projecting lens assembly at the position corresponding to the life size reproduction mode and also at the position corresponding to the smallest magnification mode. So far as the positioning of the projecting lens assembly to any one of the lens positions respectively corresponding to the smallest and largest magnification modes is involved, no error is likely to occur because the end of the stroke of movement of the projecting lens assembly is defined by a fixed stop.
As a means for retaining the projecting lens assembly at a particular lens position substantially intermediate the opposite extreme positions respectively corresponding to the largest and smallest magnification modes, a detent mechanism has usually been employed heretofore. However, it has been found that in the conventional detent mechanism, a relatively large physical force is required not only for stopping the projecting lens assembly at the particular position, but also to release the projecting lens assembly from the particular position in readiness for the movement towards the next lens position.
Where a stop is utilized as a means effective to halt the projecting lens assembly being moved in one direction, it has been found that, in an electrophotographic copying machine having three or more magnification modes, a relatively complicated mechanism is required to operate the stop depending upon the initial position from which the projecting lens assembly is moved incident to a change of the magnification and this often involves the problem that a relatively long time is required to move the projecting lens assembly.
As a means for stopping the movement of the projecting lens assembly, it is possible to use a brake of a type generally employed in a machine tool. However, this type of brake is generally bulky and expensive and, therefore, when used in an electrophotographic copying machine, it tends to increase the manufacturing cost of the copying machine.